When receiving digital broadcasts on a television located in a room using an antenna mounted outdoors, a feeder cable is necessarily routed from the antenna to the television. For that reason, when moving a television to be located to another room, a problem arises in that it takes an effort to re-route the feeder cable having been routed.
Hence, instead of using an antenna mounted outdoors, an indoor antenna has been used or a rod antenna has been built into a television itself.
To watch digital broadcasts, however, a reception signal received by an antenna is needed to exceed a certain level of strength. A problem arises in that, if such a strength level cannot be ensured, no pictures and no sounds are reproduced at all. Moreover, in a terrestrial digital broadcasting system, all the carriers within the bandwidth (5.6 MHz) must be received and a transmitter station is restricted in its output power. Accordingly, in a case of an antenna being mounted in a bad condition, there has been a problem in that it is not easy to ensure a reception signal more than a certain level of strength by the antenna.
As a measure for the above problems, it is considered that a diversity scheme, which enhances a receiver gain by diversity-combining reception signals obtained by using a plurality of antennas, is applied to a television. In diversity reception, it has been proposed that a scheme is applied to a vehicle-mounted television, in which a larger level signal is selected from those received by a plurality of antennas or a plurality of reception signals are combined (for example, refer to Japanese Patent application Publication No. 2005-223510, pp. 7-10 and FIG. 3).
Moreover, when an indoor antenna is mounted in a room or a television with a rod antenna built thereinto itself is located in a room, they must be mounted or located to be adjusted in the direction of the antenna or the television. Receiving condition for digital broadcast radio waves always varies according to weather, temperature, and position of persons in a room. For that reason, in order to keep a good receiving condition, a viewer bothers to adjust the direction of the antenna or the television itself every time the viewer recognizes degradation of the receiving condition.
As a measure for eliminating the adjustment, a scheme is applied to a television, in which antennas each are arranged on mutually orthogonal surfaces out of a plurality of television cabinet surfaces to select a larger level signal from those received by the antennas (for example, refer to Japanese Patent Application Publication No. 2006-60558, pp. 5-7 and FIG. 5).
Whereas a terrestrial digital broadcasting system uses horizontally polarized radio waves, a mobile phone system uses vertically polarized radio waves. Hence, in a mobile phone that is not a television but is capable of receiving terrestrial digital broadcasts, in order to obtain a good receiving condition for both radio waves having different polarization planes, a method is applied to a mobile phone, in which the mobile phone is provided with two antennas fixed to the casing thereof to be arranged orthogonally to each other and with an angle sensor capable of measuring the inclination of the mobile phone casing with respect to the horizontal plane (the ground), whereby the phase of a signal received by either one of the two antennas is controlled for a diversity combining (for example, refer to Japanese Patent Application Publication No. 2005-318407, pp. 4-6 and FIG. 1).
In a diversity receiver employing such conventional diversity schemes that are applied to televisions described above, for example, when a plurality of antennas are mounted on a vehicle for an on-board television, a sufficient distance between the antennas is easily ensured for suppressing a fading correlation between the antennas, so that a high diversity gain can be obtained. On the other hand, when antennas are built into a television, it is hard to ensure a sufficient distance between the antennas. Accordingly, there has been a problem in obtaining a high diversity gain.
Moreover, when antennas are built into a television, even though the antennas each are arranged on television cabinet surfaces orthogonal to each other to apply a scheme that selects a larger level signal from those received by the antennas, only a contribution from either one of the antennas is obtained, which has caused a problem that only a low diversity gain is expected.
The above described method applied to a mobile phone that is capable of receiving terrestrial digital broadcasts can be expected to obtain a good receiving condition for radio waves having polarization planes different from each other. To this end, however, antennas must be adjusted in their directions of radiation pattern to be set in the direction of an incoming terrestrial digital broadcast radio wave.
For that reason, there has been a problem in applying the method to a stationary television whose direction is not easily adjusted due to it being located in a room, whereas there arises no problem for the case with the mobile phone because a viewer can readily vary the direction of its antennas with the phone held by hand.